Safin clearly had a roller coaster career from 00 to 05 and then finally declined at a very fast pace, didnt fight hard enought to stay good at an old age.

Nalbandian probably had his best years 05-07, seems to appear out of nowhere to go on winning streaks against top players at certain periods, he started to decline in 08.

Roddick in my opinion was at his absolute best in 03-04 and never found his game since he started to experiment with a whole new approach to challenge Federer in 05 forward. He was still good in 06, definetly was declining at a fast rate in 07.

Hewitt was very good between 01-04 with one bad year in between, started to decline slowly in 05 but was still very good (like Federer 07) but in 06 he was no longer a top player.

Davydenko reminds me of Nalbandian suddenly having short periods where he plays great tennis and disappears for long periods of time, he appeared from nowhere in 06 and became a top player very fast and produced some great results every year from 06-09 even if every year he seemed to be gone during a big part of the year. Definetly declined in 2010 after finding his absolute peak in late 09 and early 10, after his choke against Federer in AO the old Davydenko was never to be seen again on court.

__________________All things are subject to interpretation whichever interpretation prevails at a given time is a function of power and not truth.

Federer started declining in 2007, fell quite a bit in 2008, but regained his form in 2009 and then started declining slowly since then. He's probably got the slowest decline I've ever seen It's incredibly hard to see the difference between 2010 Federer and 2012 Federer

Nadal isn't really in decline in my opinion. He was in 2011 in 3 slam finals and in many finals of ATP 1000 master tournaments. The problem wasthat he lost to Nole every time. If Djokovic hadn't play so well in 2011, Nadal would probably have won 3 slams last year and still be number 1. Probably Nadals game in 2010 was better, but 2011 wasn't that bad.

Nadal isn't really in decline in my opinion. He was in 2011 in 3 slam finals and in many finals of ATP 1000 master tournaments. The problem wasthat he lost to Nole every time. If Djokovic hadn't play so well in 2011, Nadal would probably have won 3 slams last year. Probably Nadals game in 2010 was better, but 2011 wasn't that bad.

If Nadal wasnt declining in 2011 then should I say Federer was declining in 2007 when he won 8 titles and 3 slams?

Hewitt declined in the mid-2000s but the reality is that the game passed him by more than he just straight declined. Even today, he's proven he could probably be a top 20-30 player if he could play a full season.

It's what the definition of decline is. If player X plays great tennis as good as ever, but a competitor improves strongly. Than I don't really consider it a decline of player X. Than I think that his competitor has improved, but not necessarily that player X declined. I consider a decline when the personal quality of tennis of player X goes strongly down.
Although you might also look from a different perspective indeed: looking purely to the rankings and statistics. And than it is true that player X was on the rankings surpassed, because his competitor improved so much. So on the rankings/statistics he declined indeed.

So the question is:
Should decline be based on the moment that the quality of tennis of somebody's own personal game goes down?
Or should decline be based on the time when he starts to fall in the overall rankings?
I would go for the first definition. But you can discuss about it.

Nadal hasn't declined, he just was barely stopped at Australia and can beat practically every player on tour. if he has declined then that would mean the tour was so horrible right now and that Nadal's best is way better than any other current players, which isn't exactly true.

absurd to assume Nadal has declined. he does everything as well as before. give it a year and if he has failed to win some titles and even seems hindered on clay then we will all have to agree. currently no signs point to any decline. the fact that Nadal is mentioned by the OP in this makes me not even want to address the other decline points